Link Directly to a Message
The following links will allow you to link into a user’s mailbox from another application. They will recognize whether the user is using beta mail or classic mail.
If you are not logged in already, it will force a login.
Composing a New Message
http://mrd.mail.yahoo.com/compose
Going Directly to a Message
http://mrd.mail.yahoo.com/msg?mid=[Yahoo! Mail message id]&fid=[Yahoo! Mail folder ID]